In pipelines carrying liquid hydrocarbons flowing at high pressure, it is known to use a monitoring device mounted externally on the pipeline to sense the pressure of the fluids flowing through the pipeline. If the pressure of the fluids flowing through the pipeline becomes excessive due to freezing of the pipeline during winter conditions or to waxing off, the pipeline may burst. Accordingly, it is known to use pressure cut-off switches mounted on the pipeline whereby the pressure of the fluids can be directly monitored.
The cut-off switches are designed such that when the pressure of the fluids exceeds a predetermined threshold, a diaphragm in the pressure switch housing is deflected by a predetermined amount. This diaphragm is typically operatively connected to a rod that moves upon the deflection of the diaphragm such that it will contact a micro-switch contained in an explosion-proof enclosure that forms part of the pressure switch and trip the switch. The pressure switch can be configured to operate upon a predetermined high or low-pressure condition as well known by those skilled in the art. Tripping of the micro-switch then sends an electrical signal to either start up or shut down a motor or pump or send the signal directly to a control monitoring system that can alert the pipeline operators of a high or low-pressure condition.
Known pressure switch housings are typically manufactured having the explosion-proof enclosure threaded onto a body that forms part of the pressure switch housing. The enclosure is then further secured to the body with setscrews. It is important to keep the enclosure fixed to the body given that the activating rod within the enclosure travels a very small amount. This travel distance is typically in the range of fifteen-thousandths of an inch (0.015″). Over time, pressure switches may need to be repaired and are, therefore, removed from the pipeline to be repaired or, in other cases, simply removed to different locations on the pipeline itself.
When pressure switches are removed from a pipeline, the force used by field personnel to remove the enclosure, if excessive, may cause the enclosure to rotate with respect to the body. This may change the calibration of the micro-switch given that the actuating rod moves only a very small distance in order to operate the micro-switch. If the enclosure is sufficiently moved with respect to the body such that the micro-switch is moved beyond the actuation range of the actuator rod, the micro-switch may not operate. In this situation, the pressure switch will not detect a high or low-pressure condition.
It is, therefore, desirable to have an explosion-proof enclosure for a pressure switch where the explosion-proof enclosure will not move relative to the body of the pressure switch itself.